In the manufacture of plastic-coated Wire, it is common practice to run the wire through an orifice, while molten plastic flows through the orifice in the same direction and around the wire. The wire, which is now coated with molten plastic, may be passed through a bath which hardens the plastic, either by cooling it or subjecting it to a chemical hardening treatment. In any case, there is some difficulty in introducing molten plastic to the orifice, because of the fact that molten plastic is most effectively compounded and heated in a screw-type extruder and because the wire cannot practicably be passed through a bore in the extruder screw. Therefore, there has developed a class of devices called "crossheads" that receive the molten plastic from the extruder and cause it to flow at a right angle to the screw axis along the line of the wire movement and of the orifice axis.
Unfortunately, this process means that one must bring about the flow of plastic at a right angle to its original flow and do so in a device that performs other functions, such as guiding the wire carefully through the orifice. If, for instance, the molten plastic flows faster or at greater pressure through the orifice on one side of the wire, as compared with the other side, thickness of the coating will differ around the circumference. Attempts have been made to solve this problem by guiding the wire off-center of the orifice to compensate, but it can be seen that variations of temperature of the molten plastic from time-to-time will produce changes in viscosity and necessitate changing the geometry between the wire and the orifice surface. It is difficult, of course, for an operator to keep up with such changes and, furthermore, the coating thickness may "drift" away from specification before the change is discovered and corrected.
One solution to the difficulty has been to lock the wire in place within the orifice at the exact center and then to use various means to maintain the flow of molten plastic constant around the circumference by using flow-directing surfaces. One way that has been used to provide such surfaces involves providing a holder surrounding and supporting the wire-guiding tube with deflector surfaces. A fairly successful version of this practice involved forming the deflector surfaces as a curved radial cut that starts at a position on the holder adjacent the passage and curves in both directions around the holder while being directed longitudinally toward the exit of the tube. This guides the molten plastic that arrives from the passage toward the side of the holder opposite the passage. This has a tendency to encourage the plastic to move to that other side, so that the natural tendency of the plastic to flow immediately to the exit on the passage side is inhibited. Even with such a construction, the plastic coating can be applied evenly on all sides of the wire only by painstaking adjustments of the control elements of the crosshead, and there is a tendency to obtain weld lines where various flows of plastic come together on the wire. These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.
It is, therefore, an outstanding object of the invention to provide a crosshead in which an even coating of molten plastic can be applied to the surface of a wire without constant adjustment of the elements.
Another object of this invention is the provision of a crosshead in which a wire coating is produced which minimizes weld lines produced by separated flows in the orifice.
A further object of the present invention is the provision of a crosshead whose interior flow surfaces are configured to give even plastic flow around the exit orifice without crossing of lines of flow.
A still further object of the invention is the provision of a crosshead deflector which is simple and rugged in construction, which can be readily manufactured, and which is capable of a long life of useful service with a minimum of maintenance.
It is a further object of the invention to provide a crosshead which can be used to give an even plastic coating on wire with little attention from the machine operator and which is easily disassembled, cleaned, and re-assembled with very little downtime.
Another object of the invention is the provision of a deflector which gives even plastic flow around the circumference of the deflector, so that the operator does not need to make adjustments in other elements, such as the tip and die.
With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.